A student said, The Earth moves around the Sun. Class 9

A student said, The Earth moves around the Sun. Class 9

Question 1.

A student said, “The Earth moves around the Sun”. In this context, discuss whether an object kept on the Earth can be considered to be at rest. Class 9

Answer:

Rest and motion are relative and depend on the reference point. If the Sun is taken as the reference point, the Earth is moving around it, so an object on Earth is also in motion. However, if the Earth is taken as the reference point, the object does not change its position and is considered at rest. Thus, an object can be at rest with respect to one reference point and in motion with respect to another.

Question 2.

The velocity-time graph from 0 s to 120 s for a cyclist is shown in Fig. Shade the areas (in different colours) representing the displacement of the cyclist Class 9

(i) while cyclist is moving with constant velocity.

(ii) when the velocity of cyclist is decreasing.

Also, calculate the displacement and average acceleration in the 120 s time interval.

Answer:

Shading:

(i) The displacement during constant velocity is represented by the rectangular area under the graph from 20 s to 100 s.

(ii) The displacement when velocity is decreasing is represented by the triangular area from 100 s to 120 s.

Calculating displacement:

= Area of triangle (0-20 s) + Area of rectangle

(20 - 100 s) + Area of trapezium (100 - 120 s)

= (1/2 × 20 × 3) + (80 × 3) + (1/2 × (3 + 2) × 20)

= 30 + 240 + 50

= 320 m

Calculating average acceleration over 120 s:

Average acceleration = (v - u)/t

= (2 - 0)/120

= 1/60 m/s² ≈ 0.017 m/s²o